Distance measuring devices that have been developed in recent years (distance measuring devices using a range image sensor) illuminate an object with an electromagnetic wave having a predetermined frequency and measure the distance to the object based on a reflected wave from the object, and form a range image using pixel values representing the measured distance (such an image is referred to as a “range image”). Some of such distance measuring devices use a plurality of cameras or perform laser scanning, whereas some others use the time-of-flight (TOF). Distance measuring devices using the TOF (see, for example, Patent Citations 1 to 3) are compact and achieve relatively high precision.
With the TOF technique, a target space for distance measurement is illuminated with intensity-modulated light, and a phase difference of the reflected wave is detected to measure the distance. The intensity-modulated light used in the TOF distance measurement may be, for example, a sine wave, a triangular wave, or a pulsed wave. The basic principle of the TOF technique is the same irrespective of which kind of light is used as the intensity-modulated light.
FIG. 1 is a diagram describing the principle of the TOF technique. A phase difference Δψ between the intensity-modulated light used to illuminate the measurement-target space and the reflected wave (reflected light) of the illuminating light is calculated using the formula below:
                    Δψ        =                              tan                          -              1                                ⁡                      (                                                            A                  2                                -                                  A                  0                                                                              A                  1                                -                                  A                  3                                                      )                                              Formula        ⁢                                  ⁢        1            where T is the modulation cycle of the intensity of the intensity-modulated light, ω is the angular frequency, and A0, A1, A2, and A3 are the intensities of the reflected wave at timings 0, T/4, T/2, and 3 T/4, respectively.
Based on this, the distance L is calculated using the formula below:
                    L        =                              c            ⁢                                          Δ                ⁢                                                                  ⁢                t                            2                                =                                    c              ⁢                                                          ⁢              Δψ                                      2              ⁢              ω                                                          Formula        ⁢                                  ⁢        2            where c is the speed of light, and Δt is the round-trip time of the light, which is written as Δt=Δψ/ω.
However, the TOF distance measurement has a tradeoff between the precision of the distance (the precision of the distance measurement) and the detectable maximum distance. More specifically, the precision of the distance is directly proportional to the modulation frequency of the intensity-modulated light used for distance measurement, whereas the detectable maximum distance is inversely proportional to the modulation frequency of the intensity-modulated light. As the intensity-modulated light with a higher modulation frequency is used for distance measurement, the precision of the distance will increase but the detectable maximum distance will decrease. In other words, the detectable distance will increase but the distance precision will decrease as the intensity-modulated light with a lower modulation frequency is used for distance measurement.
The TOF distance measuring method has this tradeoff because the TOF method fails to detect a phase difference corresponding to or exceeding one modulation cycle of the intensity-modified light used for distance measurement.
Techniques that overcome this disadvantage are described in Patent Citations 4 and 5. To enable a phase difference corresponding to or exceeding one cycle to be detectable, the techniques use multiple-frequency modulated light (intensity-modulated light generated by superimposing (multiplexing) a plurality of light elements having intensities modulated at a plurality of different frequencies, or intensity-modulated light that has been intensity-modulated using a modulation signal generated by superimposing a plurality of frequency elements). A method (technique) for detecting a phase difference corresponding to or exceeding one cycle using the multiple-frequency modulated light will be hereafter referred to as a “multiple-wave TOF technique”. With the multiple-wave TOF technique, the distance to be measured is first determined roughly (with a low precision) using low-frequency intensity-modulated light, or a light element that has been intensity-modulated at a low frequency, and then the distance is measured precisely (with a high precision) using high-frequency intensity-modulated light, or a light element that has been modulated at a high frequency. The multiple-wave TOF technique enables the distance to be measured at a precision corresponding to a high frequency (to be performed at a high precision) even when a reflected wave of the high-frequency element (the light element that has been intensity-modulated at a high frequency) from the object in the multiple-frequency modulated light has a phase difference (a high-frequency phase difference) corresponding to or exceeding one cycle of the high frequency. In this manner, the multiple-wave TOF technique solves the above tradeoff problem, and improves the precision of the distance measurement and also lengthens the detectable distance.
Another method described in Patent Citation 6 uses low-frequency light (light that has been intensity-modulated at a low frequency) and high-frequency light (light that has been intensity-modulated at a high frequency) alternately, instead of using a multiple wave (multiple-frequency modulated light). This distance measuring method has a lower distance precision than the above method that uses a multiple wave.
Citation List
Patent Literature
Patent Citation 1: Japanese Patent No. 3906858
Patent Citation 2: Japanese Patent No. 3840341
Patent Citation 3: Japanese Unexamined Patent Publication No. 2007-121116
Patent Citation 4: Japanese Unexamined Patent Publication No. S58-66880
Patent Citation 5: Japanese Unexamined Patent Publication No. H5-264723
Patent Citation 6: Japanese Unexamined Patent Publication No. H11-160065